Turbochargers for gasoline and diesel internal combustion engines are known devices in the art for pressurizing or boosting the intake air stream, routed to a combustion chamber of the engine, by using the heat and volumetric flow of exhaust gas exiting the engine. Specifically, the exhaust gas exiting the engine is routed into a turbine housing of a turbocharger in a manner that causes an exhaust gas-driven turbine to spin within the housing.
The exhaust gas-driven turbine is mounted onto one end of a shaft that is common to a radial air compressor impeller mounted onto an opposite end of the shaft. Thus, rotary action of the turbine also causes the air compressor impeller to spin within a compressor housing of the turbocharger that is separate from the turbine housing. The spinning action of the air compressor impeller causes intake air to enter the compressor housing and be pressurized or boosted a desired amount before it is mixed with fuel and combusted within the engine combustion chamber.
It is known to attach both the compressor impeller and the turbine wheel onto a common shaft by boring holes through the compressor impeller and the turbine wheel. The common shaft is extended through the bores. Fasteners are attached to the outside ends of the common shaft to prevent the compressor impeller and the turbine wheel from traveling off of the common shaft.
It is also known to attach both the compressor impeller and turbine wheel to respective ends of the common shaft without boring holes through the compressor impeller and the turbine wheel. In such “boreless” turbocharger design, the compressor impeller and the turbine wheel are configured having partial bores disposed therein that are threaded to complement and permit threaded attachment with corresponding threads on the ends of the common shaft.
A known problem with such boreless turbochargers is that oftentimes the compressor impeller is made out of aluminum and, if overspun, the aluminum-threaded area within the bore can fail. Such failure is known to occur because in conventional design, the threaded portion of the bore is the thinnest portion of the bore and, for that reason, the portion that is most prone to stripping and/or stress cracking.
Accordingly, upon failure, it is possible for the threads in the compressor impeller bore to become stripped. When this occurs, the shaft wheel assembly is no longer secured within the turbocharger and is free to come out of the turbine discharge. If the shaft wheel assembly is no longer secured, then the high inertia of the rotating assembly may create a potential hazard.
Therefore, it would be desirable to construct a compressor impeller and common shaft assembly for a boreless turbocharger configured in a manner that prevents failure, and that prevents a failed compressor impeller from coming off of the common shaft, thereby retaining the common shaft in its position within the turbocharger.